As an improvement over the spray painting of articles such as automobile bodies, boats and household appliances, a new kind of paint-coated sheet material (also called paint-coated film) has been developed to provide protective and decorative finishes. The new material comprises a flexible, stretchable, thermoplastic carrier film which has a protective and decorative paint layer of uniform thickness and appearance on one side, and, optionally, an adhesive layer on the other side. It can also have other layers such as a tie or bonding layer between the paint layer and the carrier film. In a preferred embodiment, a transparent topcoat covers the paint layer to provide an attractive basecoat-clearcoat appearance. Especially desirable is a paint-coated film having a so-called metallic paint layer which contains light-reflective flakes of metal, mica or the like.
Using known vacuum thermoforming procedures, these paint-coated films can be stretched and bonded to an article such as an automobile body panel to give a basecoat-clearcoat finish. Advantages of using such paint-coated films instead of spray painting the automobile include improved control of evaporating solvents, which reduces air pollution, and the reduction of paint sludge as a hazardous waste product. Furthermore, the new paint-coated film has a remarkably smoother and more attractive appearance than spray painted finishes. This new type of thermoformable sheet material and a process for its manufacture are described in the U.S. patent application of G. G. Reafler, Ser. No. 116,426, filed Nov. 3, 1987 now abandoned, and PCT International Publication Number WO 89/04251 of May 18, 1989. The process involves applying the paint composition to the surface of the thermoplastic carrier film by laminar flow coating, followed by drying and then coating and drying each additional layer in sequence to obtain a paint-coated film of excellent gloss and smoothness.
The laminar flow coating technique is especially advantageous in orienting any light-reflecting flakes in the paint layer in a relationship parallel to the carrier film with the result that the basecoat-clearcoat finish has a high degree of geometric metamerism, also referred to as goniochromatism or "flop". This is the ability of a paint film to exhibit subtle variations in color and appearance depending upon the directions from which it is illuminated and viewed. The effect is produced by light reflective flakes, e.g., metal flakes, incorporated in the paints. For further discussion of the optical mechanism of "flop", see the patent to Short, U.S. Pat. No. 4,199,489 and the patent to Alman, U.S. Pat. No. 4,479,718.
The aesthetics of paint-coated films are also influenced by reflected image sharpness and shininess. These characteristics are known in the paint industry as "distinctness of image" (DOI) and "gloss", respectively.
Although metallic paint-coated films offer great promise as substitutes for the spray-painting of automobile parts and other three-dimensional substrates, the fact that the films must stretch during thermoforming does present problems. It appears that when the film stretches, the light-reflective flakes in the paint layer rearrange in such a way that the film tends to lose some of its valuable surface properties such as flop, gloss and DOI, the loss being greater as the degree of stretching increases.
The present invention offers a solution to the problem of loss of flop, gloss and DOI in stretched areas of metallic paint-coated films. It can be used alone or in combination with other procedures to reduce the loss of such properties.